In Intelligent Transport Systems (ITS), it has been widely demanded a radar system utilizing radio wave such as an on-vehicle radar system. It has recently been developed, and partly commercialized, a radar system utilizing radio wave of 76 GHz band.
An on-vehicle radar system is provided on the front side of a vehicle body so that it can measure the direction, distance and relative velocity of a car running ahead of the vehicle body to function as a sensor for preventing a car accident. It will be strongly demanded that an auto cruise control (ACC) system controls the speed of a vehicle and assures a safe distance between cars. It is thus required to detect obstacles on the sides of the vehicle, such as passing vehicles and a human, for the lane selection and obstacles behind the vehicle to assist in parking and prevent car accidents, as well as obstacles ahead of the vehicle body. It has been thus increasingly demanded to provide many radars on the vehicle body.
As shown in FIG. 1, a prior millimeter-wave radar system has an FM modulator, a local oscillator, a power amplifier and multiplier, an electric divider, a transmitting antenna, a receiving antenna and a mixer. Although the RF parts are composed of MMIC, the high cost is a problem to be solved for wider acceptance. Although it is demanded to equip a plurality of radar systems as described above, such equipment would be very difficult in a practical view because the cost of the RF parts are high as described above.
It is further necessary to reduce the distance of transmission as possible on the viewpoint of losses between the oscillator and electrical divider and between the transmitting antenna and mixer. Since the oscillator is thus provided near the antenna, the oscillator is required to have high reliability under severe circumstances. On the other hand, it is strongly demanded to have stability of frequency, low noise intensity and low phase noise as a radar system. Satisfying both of the above demanded properties increases the cost of the oscillator.
Japanese Patent publication No. 2002-162465A disclosed a radar system with a sub-carrier light source using a mode locking laser. According to the system, outgoing light can be divided into a plurality of routes by means of an optical fiber and an optical divider. It is thus possible to radiate multiple radio signals using a single oscillator and to considerably reduce the number of parts necessary for the RF units, so that the cost can be reduced. It is further possible to select the positions of oscillators, so as to relax the requirements for the oscillators and to further reduce the costs.